1. Field of the Invention
The present invention relates to a crown cage applied to a ball bearing for high-speed rotation and a dental hand piece including the same.
2. Related Art
In the above-mentioned dental hand piece, a rotating shaft having a grindstone attached to its end is reversibly rotated at high speeds (for example, 300,000 to 400,000 rpm) and the peripheral speed of the grindstone is increased to improve the cutting quality thereof in order to complete teeth cutting work in a short time to reduce an unpleasant feeling of a patient.
When a deep groove ball bearing is employed as a bearing for the dental hand piece or the like, a crown cage is used. As the crown cage, a crown cage having a pocket and the like formed therein by cutting processing using a cylinder made of a thermosetting resin having heat resistance as a material in order to resist sterilization work under high temperatures (for example, 135.degree. C.) within an autoclave has been conventionally put to practical use (see, for example, Japanese Unexamined Patent Publication No. 165790/1994).
In the conventional crown cage, however, cutting processing is required, whereby the number of processes is increased, and the manufacturing cost is increased.
In order to cope with the above-mentioned situation, it is considered that a crown cage for a ball bearing is manufactured by injection molding using a thermoplastic resin.
In order to prevent a gate cutting trace remaining on the surface of the crown cage which is composed of moldings from being projected from the surface of the crown cage, the shape of the crown cage is such a shape that recesses are provided in positions corresponding to a gate position (an area including the gate position). On the other hand, as a metal mold for injection molding, a metal mold having one gate is generally employed, whereby the number of recesses is one.
If the number of recesses is thus only one, the weight balance of the crown cage is worsened. As a result, the occurrence of new problems is expected. For example, the crown cage chatters at the time of high-speed rotation.